I. Field of the Invention
This invention relates generally to a rapid assembly, concentric and flexible mating stent, tissue heart valve which substantially substitutes clamping of the tissue for sewing, and more specifically, to such a valve which is configured to provide greater exposure of at least part of the tissue between the stents.
II. Background of the Invention
Several types of heart valves are presently available for use in replacing diseased or malfunctioning heart valves in humans.
One such valve is an animal tissue valve, constructed utilizing bovine tissue or porcine aortic valve tissue or the like. These valves typically must be constructed by a trained specialist in a laboratory setting well in advance of when they will be needed to replace a diseased or malfunctioning human heart valve, and then stored in an aldehyde solution until they are needed.
Although these valves have proven to have acceptable hemodynamics, they typically suffer from a durability problem, which requires that these valves be replaced after about five to ten years of use. This is a significant problem because, after a first implant of a heart valve, subsequent implants in the same area of the heart are more difficult and risky to the patient.
The durability problem in animal tissue valves arises from two sources. First, the tissue is typically treated with glutaraldehyde or the like to attenuate the antigenicity of the tissue, and this will tan the tissue to a leather-like consistency. As a result, the tissue will become more inflexible, and over time, the valve may wear out from the stress exerted on the valve by the repeated opening and closing of the valve. Second, the antigenicity of the tissue may generate an immunological response which causes the valve to calcify, rendering it inflexible and susceptible to stress. A treatment with glutaraldehyde significantly reduces, but does not completely eliminate, the host body's immunological response to the foreign tissue. Animal tissue valves are typically not recommended for children and young adults precisely because they are not durable enough for them. One theory is that the more active immunological response in children leads to more rapid calcification of these valves, which in turn, causes their reduced durability in children.
Another problem with animal tissue valves is that they require a trained specialist to sew and assemble, and also cannot typically be sewed and assembled in the operating room because of the excessive time required. As a result, they must typically be sewed and assembled in a laboratory setting well before they are needed to replace a diseased or damaged human heart valve. Special facilities are therefore required to process and store the tissue before valve assembly, and to store the assembled valves until they are needed in the operating room. All these factors increase the cost of these valves to the patient.
Another type of valve which is presently available is the mechanical valve. This valve is typically constructed from nonbiological materials such as hard and durable ceramics, metals, and plastics and the like, and therefore, does not suffer from the durability problem associated with animal tissue valves. Because of the nonbiological nature of these valves, however, blood clots and the like can easily form on these valves, with the attendant risk to the patient that the clot fragments could break loose into the arteries, causing an embolism or stroke. This characteristic of mechanical valves is known as thromboembolism. As a result, a patient into which a mechanical valve has been implanted is required to take anticoagulants.
Anticoagulants, however, introduce another set of problems. First, it may be inconvenient for a patient to take anticoagulants. Second, any anticoagulant can lead to hemorrhagic complications in some patients, particularly older patients, with the result that mechanical valves may not be recommended for these patients. Third, some patients may be unreliable about taking their medication, especially in remote areas.
Mechanical valves are also sometimes constructed from expensive material in short supply such as pyrolytic carbon. This factor also increases the cost of these valves to the patient.
As a result, both animal tissue and mechanical valves have not proven to be entirely satisfactory, and other valve types have been explored.
Homograft tissue valves have also become available. These valves have not proven to be entirely satisfactory. Specific limitations of these valves include lack of general availability, antigenicity of the tissue, durability being no better than for animal valves, the requirement of additional surgical training to implant, the requirement of special facilities to harvest and store, the lack of availability in an adequate range of sizes, and the lack of improved performance over xenografts. Homograft valves, as currently used, also have the potential for transmitting viral diseases. All these factors result in increased cost and risk to the patient.
Autogenous tissue valves, i.e., tissue valves constructed with the patient's own tissue, have also been explored. Autogenous tissue valves, however, unlike prior art valves, must, as a practical matter, be capable of being assembled during the same surgical procedure in which the patient's diseased or damaged valve is replaced. This is because these valves cannot practically be assembled in previous surgical procedures since they cannot be sized until the annulus of the patient, into which a replacement valve is to be implanted, has been exposed in the replacement surgical procedure. Although attempts have been made to size the annulus using X-rays or the like, these methods are only approximate and have not proved reliable.
Assembly during the same surgical procedure, however, requires that the autogenous tissues used in these valves be extracted, and then the valves themselves be sized and assembled rapidly to avoid any prolongation of the time that the patient is maintained on a cardio-pulmonary bypass. Typically, 10-15 minutes are required to place anchoring sutures in the valve annulus, and construction of the autogenous tissue valve should be accomplished in that period of time. The autogenous tissue is available for preparation early in the surgical procedure for preliminary preparation.
Attempts have been made to construct autogenous tissue valves in the limited time available while the patient is on the operating table. These attempts, however, were abandoned because of the difficulty encountered in constructing a durable and structurally sound valve in the limited time available, lack of a standardized repeatable method of valve assembly, and poor results with fresh, untreated tissue.
These attempts were abandoned, in part, since the valves were constructed and held together typically by the time-consuming and error-prone method of suturing the tissue to a unitary frame or stent. Not only did this procedure require too much time, i.e., more than 10 minutes, to assemble the valve, but it was also found that the risk of uncertain valve quality, caused by the rushed conditions under which the valves were assembled, was too high. In fact, poor valve quality caused one early researcher to state: "The construction of the graft in the operating theatre during surgery, with a limited amount of time available, does not offer the best conditions for the preparation of a perfect valve." See "Replacement of Heart Valves With Frame-Mounted Tissue Grafts," Ionescu et al., Thorax (1974), Vol. 29, p. 56, at p. 65. This procedure was also expensive and of limited availability, since it typically could only be performed by skilled surgeons. For all the foregoing reasons, these attempts at constructing autogenous tissue valves were abandoned.
In U.S. Pat. No. 4,470,157 (the '157 patent), Jack W. Love, one of the inventors herein, pioneered an autogenous valve which utilized mating stents to clamp the tissue between the stents. This patent did not specifically address the problem of prolapse. This problem occurs when the tissue is not uniformly distributed amongst the leaflets of the valve, the leaflets are not of uniform size, and the co-aptive edges of the leaflets do not meet uniformly during valve closure, resulting in valve leakage and undue stress on the leaflets.
Prolapse can be a significant problem, and later attempts to reduce or eliminate prolapse, such as alignment stitches or the like, have not proven successful in completely eliminating prolapse. This is because the stitches are difficult to accurately place in an operating room environment where time constraints are important.
Another problem not specifically addressed by the '157 patent was the tendency for the tissue between the stents to slip, due to irregularities in the specific tissue used in the valve interfering with the clamping force generated by the stents, and due to the lack of uniformly distributed clamping force both along the annular bases of the stents, and between the stent posts.
A further problem not specifically addressed by the '157 patent was the tendency for unexposed tissue clamped between the stents to deteriorate and even disappear.
U.S. Pat. No. 4,687,483 describes a valve which is assembled by registering a significant number of pins and studs extending from an inner frame to corresponding holes and slots in an outer frame, securing the pins with securing washers, and sewing tissue or cloth frame coverings together. Because of the large number of pins and studs involved, and the sewing required, this valve is not capable of being assembled in the limited time available in an operating room environment. Consequently, this valve is not satisfactory for rapidly assembling an autogenous tissue valve in the operating room.
U.S. Pat. No. 4,192,020 describes a valve which utilizes an adhesive such as polyurethane dissolved in tetrahydrofuran to secure fabric to wire frames. An adhesive such as this is toxic, and not suitable for affixing tissue, especially viable human tissue, to a valve. Consequently, this valve is not satisfactory either for use in assembling an autogenous tissue valve.
U.S. Pat. No. 4,501,030 describes a complex valve which utilizes a significant number of sutures to assemble the valve. Consequently, this valve cannot be assembled in the limited time available either.
In sum, for all the foregoing reasons, it is an object of the present invention to provide a rapid assembly, flexible and concentric mating stent tissue valve that substantially substitutes clamping for sewing, which is configured to generate a substantially uniform clamping force on the tissue between the stents in the assembled valve, and which is configured to generate a self-adjusting clamping force which adjusts for tissue irregularities. It is a further object to provide a valve which achieves proper alignment and prevents movement of the tissue during valve assembly to prevent prolapse. It is a further object to provide a method for assembling such a valve which is standardized and reproducible, and which can easily be learned and practiced by a non-surgeon. It is still a further object to provide a valve which provides increased exposure of the tissue clamped between the stents.
Additional objects and advantages will be set forth in the description which follows or will be apparent to those or ordinary skill in the art who practice the invention.